This invention relates to transmission controls and, more particularly, to electro-hydraulic controls for a continuously variable transmission.
Continuously variable transmissions include a continuously variable unit (CVU), such as a belt and pulley mechanism, and a gearing mechanism, such as a planetary gear arrangement. The gearing mechanism is conventionally controlled by torque transmitting mechanisms (i.e., clutches or brakes) that are selectively operated by hydraulic fluid. The continuously variable unit requires a high pressure to ensure sufficient clamping forces for the belt and pulley mechanism. The effective ratio of the CVU is determined by the radius at which the belt engages the pulleys. In most instances, the ratio can be varied from an underdrive to an overdrive.
The amount of clamping pressure required is a function of the input torque to the transmission and the ratio at which the variable transmission unit is operating. If the clamping pressure is low, there is a possibility of belt slippage, and even a small amount of belt slippage can be detrimental to the CVU. The ratio of the CVU is changed by reducing or increasing the pressure acting on one of the sheave halves of one of the pulleys, generally the input pulley, while the pressure at the other pulley is maintained substantially constant. If the control pressure is excessive at either pulley, there is an efficiency loss within the transmission and possible damage to or overstressing of the components of the CVU.
The control pressure level required to engage the torque transmitting mechanisms is generally lower than the pressure required to control the CVU. The amount of pressure required is essentially a function of torque being transmitted and size of the conventional clutch hardware, consisting of a movable piston and a clutch pack. If the control pressure is below the required value, slippage of the friction plates can occur, which will shorten the life of the torque transmitting mechanisms.
The hydraulic circuit generally includes a pressure regulating valve that must be capable of regulating the correct clamping pressure and the correct torque transmitting engagement pressure to avoid a shortened life for either the variable transmission unit or the torque transmitting unit.
The pressure within the circuit is generated by a positive displacement pump. The amount of pressure that the pump can generate is a function of the pump speed, flow demand of the transmission, and leakage within the circuits. The more flow the transmission requires, the lower the line pressure that can be generated. Generally, two regulator valves are employed, one for the CVU control and one for the torque transmitting mechanisms. The valves are usually disposed in flow relation such that the CVU line pressure circuit has priority. All of the hydraulic fluid not used by the CVU control is passed to the regulator valve for the gearing section controls. Thus, the regulator valve for the gearing section controls must be sized to accommodate large amounts of fluid flow at times during the operation of the transmission, particularly during ratio changes when the pressure at the control pulley is being reduced. This means that the flow priority is set in such a way that if the pump is not able to supply the requested line pressure for the CVU, part of the transmission flow demand is reduced through the regulator valve in order to achieve the desired line pressure.
For example, in order to preserve the required line pressure to the sheaves in a belt and pulley type transmission, the line pressure regulator valve may reduce the flow to the oil cooler. The oil flowing to the sheave is said to have the higher priority than the oil going to the oil cooler. Generally, this sacrifice of flow to one part of the transmission in order to maintain the pressure in another part of the transmission should happen only during extreme or transient conditions, such as the development of a large leak or a rapid ratio change within the transmission.
It is an object of this invention to provide an improved hydraulic control system for a continuously variable transmission.
In one aspect of the invention, the hydraulic control system regulates the line pressure to the CVU pulleys, clutches, solenoids and torque converter of a continuously variable transmission.
In another aspect of the present invention, separate regulator valves provide a first line pressure and a second line pressure to thereby minimize oil flow demand and improve the transient shift performance and fuel economy.
In yet another aspect of the present invention, excess pump flow is only directed to the pressure regulator controlling the first line pressure which enables the remaining valves within the control system to be reduced in size and weight.
In still another aspect of the present invention, the hydraulic control system prioritizes the pressure in the first line pressure above all other pressure demands and directs this line pressure to the sheaves of the continuously variable transmission.
In still another aspect of the present invention, the hydraulic control system minimizes the effects of transient flow demands and the pressure output of the solenoid controls by prioritizing the first line pressure above all other pressure demands.
In a further aspect of the present invention, the control system minimizes slippage within the torque transmitting mechanisms by prioritizing second line pressure flow to the clutches above cooler flow demands.
The present invention employs a single variable bleed solenoid (VBS) valve which controls both the first line pressure regulator valve and the second line pressure regulator valve. The use of a single variable bleed solenoid to perform this function reduces the cost of the transmission control. The hydraulic system has an actuator feed limit valve which protects the variable bleed solenoid valve from over pressurization. A second line pressure feed limit valve is incorporated within the control to limit both the torque transmitting mechanisms and the torque converter from over pressurization by limiting the output of the second line pressure regulator valve prior to distribution of fluid to either of these devices.